Subterranean boreholes are typically drilled using a drill string which includes drill pipe or coiled tubing, with a drill bit connected at the distal end of the drill string.
The drill bit may be rotated by rotating the entire drill string, and/or the drill bit may be rotated by a downhole motor which is included as a component of the drill string.
A typical form of downhole motor is a progressing cavity motor. A progressing cavity motor includes a power section comprising a stator and a rotor received within the stator. Drilling fluid is passed through the power section of the downhole motor in order to convert fluid energy into rotational energy of the rotor within the stator. The rotor is typically connected indirectly with the drill bit via a drive shaft.
In a progressing cavity motor, the rotor both rotates and nutates within the stator. As a result, the drive shaft is typically connected with the rotor via a drive connection such as a flex shaft or a constant velocity coupling, which accommodates and assimilates the nutation of the rotor so that the drive shaft and the drill bit rotate without nutating.
The rotor, the drive connection and the drive shaft provide a drive train which is typically received within a housing of the downhole motor. A distal end of the drive shaft may be connected directly with the drill bit (if the downhole motor is located at the distal end of the drill string), or may be connected with the drill bit via other components of the drill string (if the downhole motor is not located at the distal end of the drill string).
In a typical progressing cavity motor, a bearing section is typically provided axially between the drive connection and the drill bit, typically along the drive shaft. The purpose of the bearing section is to transfer forces between the housing and the drive train of the downhole motor, while enabling the drive train to rotate within the housing. The location of the bearing section (distal of the drive connection) ensures that the bearing section will not be subjected to undue wear due to nutation.
A typical bearing section may include one or more thrust bearings and one or more radial bearings. The thrust bearings transfer axial loads between the housing and the drive train. The radial bearings transfer radial loads between the housing and the drive train.
Axial loads experienced by a downhole motor may include on-bottom loads or off-bottom loads. On-bottom loads are typically compressive loads such as reactive forces exerted on the drill bit by the end of the borehole. Off-bottom loads are typically tensile loads and may result from drilling fluid passing through the motor (especially through the power section and through the drill bit) and/or from the weight of the motor and components of the drill string which are distal of the motor.
As the names suggest, on-bottom loads are typically most important during drilling when the drill bit is engaged with the end of the borehole, while off-bottom loads are typically most important when the drill string is raised above the end of the borehole so that the drill bit is not engaged with the end of the borehole.
Radial loads experienced by a downhole motor may be due to side loading on components of the motor or transverse vibration of components of the motor.
A downhole motor therefore typically includes in the bearing section one or more “on-bottom” thrust bearings, one or more “off-bottom” thrust bearings, and one or more radial bearings.
Locating all of these bearings in the bearing section, and locating the bearing section below the drive connection may result in a relatively long distance between the drive connection and the distal end of the downhole motor. This long distance is generally undesirable for several reasons.
First, a relatively long distance between the drive connection and the distal end of the motor results in a relatively long distance of the drive train between the power section and the drill bit. This relatively long distance can result in earlier failure of a downhole motor in comparison with a downhole motor which has a relatively short distance between the power section and the drill bit, due to the effects of torque and torsion on the drive train.
Second, bent downhole motors may be used for directional drilling, since the drilling direction can be controlled by controlling the orientation of the bend if the drill bit is rotated only by the motor. The bend in a bent downhole motor is typically located adjacent to the drive connection. As a result, a relatively long distance between the drive connection and the distal end of the motor provides a relatively long “bend to bit” distance. A relatively long bend to bit distance is disadvantageous for directional drilling because a longer bend is inherently less stiff than a shorter bend, with the result that a bent downhole motor with a relatively short bend to bit distance tends to be able to provide a larger “build angle” than a bent downhole motor with a relatively long bend to bit distance.
Third, bent downhole motors may be used for non-directional drilling by rotating the drill string in addition to or in substitution for rotating the drill bit with the motor. Rotation of the drill string rotates the motor, which results in rotation of the bend in the motor. The longer the bend to bit distance of the bent downhole motor, the greater the side loads and bending moments which are exerted on the motor during non-directional drilling. These side loads and bending moments may contribute to failure of the motor. In addition, the longer the bend to bit distance of the bent downhole motor, the larger the diameter of the borehole which will be drilled by the drill string. Although a large diameter borehole may be desirable in some circumstances, the use of a bent downhole motor with a relatively long bend to bit distance for non-directional drilling may be relatively inefficient in most circumstances.
As a result of the disadvantages associated with a relatively long distance between the drive connection and the distal end of a downhole motor, it would be desirable to take steps in the design and configuration of downhole motors to reduce this distance.
One opportunity for reducing the distance between the drive connection and the distal end of a downhole motor is provided by the bearing section of the motor. As one example, by moving some components of a typical bearing section away from the conventional position distal of the drive connection, the components of the bearing section can be distributed along the length of the motor. As a second example, by decoupling some or all of the loads which are imposed on the motor, such loads may be separated and apportioned amongst components of the bearing section, and the size (and length) of such components can potentially be reduced.
Some prior art approaches to downhole motors with non-conventional bearing arrangements are found in U.S. Pat. No. 6,629,571 (Downie), U.S. Pat. No. 7,416,034 (Downie et al), U.S. Pat. No. 7,802,638 (Downie et al), and U.S. Patent Application Publication No. US 2011/0147091 (Bullin).